The present invention relates to a high NA objective lens typically used for an optical head of an optical disc drive, which utilizes an optical disc such as a DVD (digital versatile disc), MO (magneto-optical) disc, or the like.
Recently, in order to meet a requirement of decreasing in size and weight of optical disc drives, plastic objective lenses, both surfaces of which are formed as aspherical surfaces, have been used in optical heads for such optical disc drives.
Although the plastic lenses have various advantages, the refractive index and/or shape of a plastic lens may easily be changed depending on changes in temperature in comparison with those of a glass lens, and due to this variable characteristics, performance of the plastic lens may vary. For example, when the temperature increases, the refractive index of a plastic lens decreases. Then, a spherical aberration of the plastic lens changes in an over-corrected direction, and a wavefront aberration increases. Generally, a ratio of the change of the refractive index of a plastic lens with respect to the change of the temperature is approximately xe2x88x9211xc3x9710xe2x88x925/xc2x0 C. Accordingly, when the objective lens of an optical head is formed with a plastic lens, the above characteristics may affect the performance of the optical head.
TABLE 1 shows the wavefront aberration (rms value, unit of which is xcex: wavelength) using an NA (numerical aperture) as a parameter when the temperature is raised by 40 degrees (i.e., the refractive index is changed by xe2x88x92440xc3x9710xe2x88x925) for a plastic lens having a focal length of 3.0 mm, working wavelength of which is 650 nm. FIG. 16 is a graphical representation of the data of TABLE 1.
Generally, the objective lens for a CD drive is designed such that the NA is approximately 0.45 and the upper limit of an allowable range of the wavefront aberration is 0.04xcex. Therefore, even if the temperature is changed by about 90 degrees, the wavefront aberration stays within the allowable range. Accordingly, for the objective lens for the CD drive, the variation of the wavefront aberration does not practically affect the performance of the objective lens.
The objective lenses for a DVD drive is generally designed such that the NA is approximately 0.60. The objective lens for an MO disc drive is designed such that the NA is approximately 0.55. The upper limit of the allowable range of the wavefront aberration for such lenses is approximately 0.03xcex. Therefore, when the temperature changes by 40 through 50 degrees, the wavefront aberration exceeds the upper limit of the allowable range, which may likely affect data recording/readout operation.
It is therefore an object of the invention to provide an improved objective lens for an optical head, the wavefront aberration of which due to the variation of the temperature is well suppressed, and is applicable, as a high NA objective lens, to the DVD drive or MO disc drive at a relatively wide usable temperature range.
For the above object, according to an aspect of the invention, there is provided a single element objective lens for an optical disc drive. The objective lens converges a laser beam, which is emitted by a laser source, on a data recording surface of an optical disc through a protective layer of the optical disc. One surface of the objective lens is divided into a central area including an optical axis of the objective lens and a peripheral area surrounding the central area. The central area is formed as a continuous surface having no stepped portions, while the peripheral area is provided with a diffraction lens structure formed by a plurality of concentric annular zones including minute steps. Further, the diffraction lens structure is configured to compensate for variation of converging characteristic of the objective lens due to a change of a temperature.
By providing the diffraction lens structure as described above, the affect of the temperature variation can be avoided. It should be noted that, in practical use, the diffraction lens structure may be provided only in the peripheral area, and the variation of the converging characteristic can be suppressed in an allowable range.
Preferably, area of the peripheral area is not greater than area of the central area. If the peripheral area becomes greater than the central area, the wavelength dependence of the spherical aberration may become too great, and the performance of the objective lens may be worsened due to individual differences of wavelengths among laser sources.
Optionally, the diffraction lens structure has a characteristic such that the spherical aberration thereof changes in an under corrected direction when a wavelength of the laser beam incident on the objective lens increases.
It should be noted that, if the objective lens is formed of plastic, the refractive index of which is lowered when the temperature increases. On the other hand, a wavelength of the laser beam increases when the temperature increases. Therefore, due to a balance between the change of the spherical aberration of the refractive lens due to the change of the temperature and the change of the spherical aberration of the diffraction lens structure due to the change of the wavelength, which is caused by the change of the temperature, the variation of the converging characteristic of the objective lens due to the temperature change can be compensated.
According to another aspect of the invention, there is provided a single element objective lens for an optical disc drive, the objective lens converging a laser beam emitted by a laser source on a data recording surface of an optical disc through a protective layer of the optical disc. One surface of the objective lens is divided into a central area including an optical axis of the objective lens and a peripheral area surrounding the central area. A diffraction lens structure formed by a plurality of concentric annular zones including minute steps is provided only in the peripheral area. The diffraction lens structure is configured to compensate for variation of converging characteristic of the objective lens due to a change of a temperature.
Optionally, the objective lens is formed of plastic, refractive index of which is lowered when the temperature increases. Further, a wavelength of the laser beam increases when the temperature increases.
According to another aspect of the invention, there is provided an optical system of an optical head for an optical disc drive, including a laser source unit that emits a laser beam, and a single element objective lens that converges a laser beam emitted by the laser source unit on a data recording surface of an optical disc through a protective layer of the optical disc. One surface of the objective lens is divided into a central area including an optical axis of the objective lens and a peripheral area surrounding the central area. The peripheral area is provided with a diffraction lens structure formed by a plurality of concentric annular zones including minute steps. The diffraction lens structure is configured to compensate for variation of converging characteristic of the objective lens due to a change of a temperature.
Optionally, the laser source unit is configured to selectively emit one of a first laser beam and a second laser beam, a wavelength of the second laser beam being longer than a wavelength of the first laser beam. The second laser beam is incident on the objective lens as a diverging beam, while the first laser beam is incident on the objective lens as a beam having less divergence than the second laser beam. The objective lens converges the first laser beam on a data recording surface of a first optical disc through a first protective layer of the first optical disc. Further the objective lens converges the second laser beam on a data recording surface of a second optical disc through a second protective layer of the second optical disc. A data density of the first optical disc is higher than a data density of the second optical disc, while a thickness of the first protective layer is smaller than a thickness of the second protective layer.
It should be noted that a wavelength of the laser beam emitted by the laser source unit may increase when the temperature increases, while the objective lens is formed of plastic and the refractive index is lowered when the temperature increases.
In this case, the diffraction lens structure is configured to have a characteristic in terms of a spherical aberration such that the spherical aberration changes in an under corrected direction when a wavelength of the laser beam incident on said objective lens increases.